1. Field of the Invention
The present invention concerns an apparatus for nondestructive measurement of a rubbing angle (alignment control direction) in a substrate just after a rubbing treatment of the substrate, as well as manufacturing methods of a liquid crystal display device and an optical film using a substrate controlled for the rubbing angle by the measurement.
2. Description of the Related Art
FIG. 12 schematically shows the structure of an IPS (in-plane switching) type liquid crystal display device as an example of a liquid crystal display device. An unpolarized light emitted from a backlight 1241 passes through a lower polarizer plate 1231 and is polarized. The polarized light passes a liquid crystal panel 1211 and, further, passes an upper polarizer plate 1212, by which a liquid crystal device displays an image. Between the polarizer plate and the liquid crystal panel, a retardation plate is sometimes provided for color compensation to the liquid crystal panel 1211 (bifringence thereof) or for improving optical characteristics such as a view angle characteristic. FIG. 12 shows the structure of a liquid crystal display device using a composite film 1211 formed by stacking a retardation plate 1213 to the upper polarizer plate 1212. A composite film 1211 has a polarization axis 1214 of the polarizer plate 1212 and a slow axis (phase retardation axis) 1215 of the phase retardation plate 1213.
The upper polarizer plate 1212 and the lower polarizer plate 1231 are bonded on both surfaces of the liquid crystal panel 1221 (respective main surfaces of a first glass substrate 1223 and a second glass substrate 1224) such that the polarization axis 1214 of the upper polarizer plate 1212 and the polarization axis 1232 of the lower polarizer plate 1231 are crossed at 90° (to constitute Crossed Nicols). The amount of light transmitting the upper polarizer plate 1212 is controlled by controlling liquid crystals 1222 of the liquid crystal panel 1221 thereby changing the polarization direction of the light passing the liquid crystal panel 1221. A so-called inverse parallel rubbing treatment of moving rubbing rollers to the directions opposite to each other along an illustrated rubbing direction 1225 is applied to respective main surfaces of the first glass substrate 1223 and the second glass substrate 1224 in contact with the liquid crystals 1222. In a period where an electric field is not applied to the liquid crystals 1222 (hereinafter referred to as a non-electric field period), the liquid crystals 1222 are arranged in accordance with the direction of an anchoring energy applied by rubbing to the substrates (substantially equal with the rubbing direction 1225). The direction of the anchoring energy is referred to as an alignment axis. An IPS liquid panel is constructed such that the alignment axis is at 0° or 90° relative to the polarization axis 1232 of the lower polarizer plate 1231. In an initial state where the electric field is not applied to the liquid crystals 1222, since the light incident from the lower polarizer plate (backlight 1241) to the liquid crystal panel 1221 is not changed for the polarization direction thereof by the liquid crystals 1222, it is interrupted by the upper polarization plate 1212 having the polarization axis 1214 crossing the polarization direction and a screen of the liquid crystal display device is displayed black. When the polarization direction of a light passing the liquid crystal panel 1221 changes by applying an electric field to the liquid crystal panel 1221 to change the alignment direction of the liquid crystals 1222, and the amount of a light transmitting the upper polarizer plate 1212 increases and the screen of the liquid crystal display device is displayed white. In the liquid crystal display, the amount of light is changed by the switching of the liquid crystals by the electric field as described above.
A relation between the anchoring energy and the twisting of liquid crystals (which changes the aforementioned alignment direction) in the liquid crystal panel is to be described with reference to FIG. 13 to FIG. 15. In the following description, one of the pair of substrate 1223 and 1224 sandwiching the liquid crystals 1222 described above is referred to as a TFT substrate in which plural pixels each having a pixel electrode and a thin film transistor (hereinafter referred to as TFT) for controlling the application of an electric field to the liquid crystals are arranged in a 2-dimensional manner, and the other of them is referred to as a CF substrate in which a color filter (hereinafter referred to as CF) is formed for providing a specific color to a light passing liquid crystals. FIG. 13 shows, in an enlarged scale, a region formed with one of the plural pixels on the main surface of the TFT substrate and FIG. 14 shows, in an enlarged scale, a region of a main surface of the CF substrate opposed by way of a liquid crystal layer to the region of the main surface of the TFT substrate. FIG. 13 shows the pixel electrode (for example, an ITO interconnection (an ITO wiring or an ITO layer) formed of indium-tin oxide) 1311, a thin film transistor 1312 for controlling the potential thereof, a drain interconnection (also described as, a drain wiring, or a drain line) 1313 for supplying an image signal to the thin film transistor 1312, a gate interconnection (also described as, a gate wiring, or a gate line) 1314 for controlling the supply of the image signal from the thin film transistor 1312 to the pixel electrode 1311, and a source interconnection (also described as, a source wiring, or a source line) 1315 for electrically connecting the thin film transistor 1312 and the pixel electrode 1311. The TFT substrate is also referred to as an array circuit substrate in view of the arrangement of TFTs for applying a voltage to the liquid crystals on every pixel on the main surface. FIG. 14 shows a color filter 1411, opposed to the pixel electrode 1311, a pair of color filters 1412 and 1413 adjacent on both sides of the color filter 1411, and a black matrix 1414 spacing adjacent color filters. Colors of the color filters 1411, 1412, and 1413 are different from each other and the color filters are formed, for example, with a G (green) color resist for the color filter 1411, an R (red) color resist for the color filter 1412, and a B (blue) for resist for the color filter 1413, respectively.
The main surface of the TFT substrate shown in FIG. 13 is rubbed along direction 1301 deviated from an interconnection direction (shown by a dashed-dotted line) at an angle of θTFT, and the main surface of the CF substrate (color filter substrate) shown in FIG. 14 is rubbed along direction 1401 deviated from the interconnection direction (shown by a dashed-dotted line) at an angle of θCF. The interconnection direction illustrated herein is defined as an extending direction of the drain interconnection 1313 at the main surface of the TFT substrate and, as the extending direction of the drain interconnection 1313 of the TFT substrate is projected to the main surface of the CF substrate at the main surface of the CF substrate. An alignment film is coated to the main surface of the TFT substrate and the main surface of the CF substrate before the rubbing treatment, and the anchoring energy is given to each of the alignment films by the rubbing treatment. Then, the TFT substrate and the CF substrate are overlapped to each other such that the rubbing directions 1301 and 1401 applied respectively to them are in inverse parallel, and liquid crystals are injected in a space separating the main surface of the TFT substrate and the main surface of the TFT substrate to prepare a panel shown in FIG. 15(a). FIG. 15(a) shows a portion of the TFT substrate and the CF substrate in which the respective main surfaces are rubbed in the directions 1301 and 1401 and the CF substrate is partially cut away such that the structure of the main surface of the TFT substrate covered with the CF substrate can be observed. Further, FIG. 15(b) shows a cross section of the panel shown in FIG. 15(a). As shown in FIG. 15(b), an alignment film 1513 is formed to the main surface of a CF substrate 1511, and an alignment film 1514 is formed to the main surface of a TFT substrate 1512 and liquid crystals 1515 are sealed between the main surfaces.
In the panel comprising the substrate rubbed in the direction deviated from the direction of axis Y in an orthogonal coordinate (that is, the interconnection direction) by a predetermined angle in FIG. 15(a), liquid crystals near the main surface of the TFT substrate are arranged along the anchoring energy applied to the alignment film 1514 on the side of the TFT substrate 1512, while the liquid crystals near the main surface of the CF substrate are arranged along the anchoring energy applied to the alignment film 1513 on the side of the CF substrate 1511. Liquid crystals 1515 present between them are arranged being twisted so as to connect the anchoring energy of the upper and lower substrates (CF substrate 1511, TFT substrate 1512). In the liquid crystal panel, since the optical characteristic such as contrast is lowered when the alignment angle of the liquid crystals is deviated relative to the polarization axis, it is important to apply rubbing at an angle as a designed value (being aligned with the axis Y in this case).
Further, as shown in FIG. 12, the liquid crystal display has optical films such as the polarization film 1212 and the retardation film 1213 and each of the optical films has an optical anisotropic axis. Upon manufacturing the optical film or bonding it to the liquid crystal panel, in a case where the angle of the optical anisotropic axis is deviated from a designed value thereof, the optical performance of the liquid crystal display is lowered. Accordingly, control for the optical anisotropic axis is important also in the manufacturing of the optical film.
Measurement for the angle of the liquid crystal alignment axis of a liquid crystal panel or an optical film includes, as a general method, a method of measuring refractive index anisotropy (retardation). This is a method of rotating a measuring object (liquid crystal panel or optical film) to a polarization axis under Crossed Nichols thereby measuring an angle at which the transmittance is lowered most. Further, as a method of inspecting a rubbed substrate, JP-A No. Hei 6-59230 discloses a technique of observing a rubbed substrate surface under Crossed Nichols and evaluating the adequacy of rubbing based on the image of streaks thereof.